Remix: Ultimate Spool Roller for Ender 3

This is a remix of https://www.thingiverse.com/thing:3341611 to make it compatible with mounting on the top rail of an Ender 3. You can re-use the same rail nuts and screws to mount this in the same location. truglodite's design is excellent, I've always preferred the dual rollers over the single, and this one works very well. Pls note that if you print really tall objects, this may affect your max Z height, reduces it by 15-20mm roughly. I was going to make an adapter that allows full z height movement, but I find that I rarely print things that tall, so I didn't bother, and it takes only about 5 mins to swap out.